Bitumen (a form of heavy oil) is commercially recovered from oil sand in Alberta. This recovery is achieved by mining the oil sand, extracting the bitumen from the oil sand in the form of froth, cleaning the froth product to remove contained water and solids, and upgrading the bitumen to produce a variety of oil products. Extraction is achieved using a process referred to in the industry as the hot water process. This process is described in considerable detail in prior art patents and in the technical literature. A short summary of the process is given below.
To better understand the hot water process, it is helpful to know about the nature of oil sand itself. Oil sand comprises grains of sand which are individually sheathed or wetted with a very thin layer of connate water. Small flecks of bitumen are located in the interstices between the water-wet grains. Minute clay particles (termed "fines") are dispersed in the connate water phase.
In general, the hot water process involves slurrying the as-mined oil sand in hot water with surfactant-forming caustic and relying on a combination of heating, agitation, and surfactant actions to disperse the bitumen from the solids and into the aqueous phase of the slurry. At this point, bitumen flecks coalesce and some become attached to air bubbles, so that the aerated bitumen floats as a froth. The froth is then recovered.
The composition of the as-mined oil sand is variable. The oil, water and solids contents of oil sand processed at applicants' plant can typically vary as follows:
TABLE I ______________________________________ oil: 6 to 18% by wt. water: 0 to 14% by wt. fines (-44 u) 10 to 75% by wt. total solids: 70 to 90% by wt. ______________________________________
This variability in composition leads to wide swings in the processability of the feed when using the hot water process. A "rich" oil sand, high in oil and low in fines, will give a high yield of good quality extraction product; a "lean" oil sand, low in oil and high in fines, will give a relatively low yield of relatively poor quality product. By way of example, Table II sets forth typical values for the composition of oil sand feed, for rich and lean ores of sufficient quality to be processed, and the recovery and composition of the primary froth product which one would typically obtain.
TABLE II ______________________________________ Lean Oil Sand: Percent Oil Recovered As: ______________________________________ Oil 6% Primary Froth 65% Water 11% Froth Composition: Fines 21% Oil 55% Solids 83% Water 34% Solids 11% ______________________________________ Rich Oil Sand: Percent Oil Recovered As: ______________________________________ Oil 12% Primary Froth 92% Water 3% Froth Composition: Fines 14% Oil 65% Solids 85% Water 27% Solids 8% ______________________________________
One of the disadvantages of producing a froth product stream having such wide swings in its compositional make-up is that the downstream equipment has to be sized to accommodate the worst case. In addition, constantly adjusting the downstream processing for optimization involves difficulty.
So any simple and effective means for reducing the water and solids contents of the froth and smoothing out the froth composition variations, would be desirable.
At this point it is useful to provide a short description, in greater detail, of the hot water extraction process, as practised at the plant of the present assignees, to put the invention into context. The process involves:
Mixing as-mined oil sand with hot water and a small amount of NaOH in a rotating horizontal drum for a period of several minutes to produce an aqueous slurry of thick consistency. Steam is sparged into the slurry to develop an exit temperature of about 180.degree. F. In this slurrying step, the lumps of oil sand are ablated, the bitumen flecks are heated and the NaOH reacts in situ with bitumen moieties to form surfactants. The bitumen flecks become liberated from the solids and are dispersed into the aqueous phase. In addition, air bubbles are entrained into the slurry. Some of the bitumen flecks coalesce and coat air bubbles; PA1 The slurry is then diluted with additional hot water and is temporarily retained under quiescent conditions in a large, cylindrical, conical-bottomed, open-topped vessel referred to as a primary separation vessel (hereafter "PSV"). In the PSV, "spontaneous flotation" of the bitumen occurs. More particularly, buoyant bitumen floats to form an oily froth. This froth, (called "primary froth"), overflows the upper lip of the PSV and is conveyed away from the vessel in a downwardly sloping, broad channel, referred to as a launder. As the froth is forming in the PSV, the coarse solids settle and are discharged from the base of the vessel. This stream of coarse solids, associated with some water and a small amount of bitumen, is called "PSV tailings". Some residual, insufficiently buoyant oil remains in the watery main body of the PSV contents - this fluid is referred to as "PSV middlings". The PSV middlings and PSV tailings are combined and are fed into a vessel referred to as the tailings oil recovery vessel (hereafter "TORV"). This is a cone settler, into which the PSV middlings and tailings are fed and are caused to move outwardly and laterally from a central feed point. The feed is contacted from below by an upwelling aerated stream of PSV middlings. A second yield of bitumen froth forms and overflows the vessel rim and is conveyed away in a launder. In the vessel, the coarse solids settle, are concentrated in the narrowing lower end of the cone, and are discharged as tailings. (The TORV process is described in greater detail in U.S. Pat. No. 4,545,892.) The process occurring in the TORV is also characterized as spontaneous flotation; PA1 As the last step in the extraction process, the middlings from the TORV are fed to a bank of induced flotation cells, in which the feed is vigorously sub-aerated and agitated and from which a third froth stream is recovered. This froth (termed "secondary froth") is cleaned by settling out some contained water and solids by temporarily retaining the mixture in a settling tank. PA1 That bitumen froth issuing from a flotation vessel, such as the PSV or TORV, contains discrete water particles ranging in size from microscopic flecks to pea-size globules; PA1 That, when in a gently sloped channel or launder, some of the water particles migrate downwardly through the froth body and collect and coalesce along the bottom wall of the channel in the form of a discrete, water-rich layer, which additionally contains some settled solids. PA1 The water-rich bottom layer is at least partly diverted through an aperture or outlet in the channel bottom wall and is temporarily retained in an upstanding container positioned beneath the channel; PA1 In addition, a weir is positioned immediately downstream of the diversion aperture. The weir extends transversely across the channel and functions to keep a small head of the dirty water with its surface above the aperture; PA1 Thus a column of fluid comprising upper and lower layers, having distinctive compositions and a discernible interface, is formed in the separator consisting of the combination of the weir and container. The upper layer is relatively clean bitumen froth, containing some water and solids, and the lower layer is mainly water containing solids and traces/ of bitumen; PA1 The elevation of the interface is monitored; and PA1 A variable pump, controlling the rate of withdrawal of fluid from the base of the container, is operated in response to the location of the interface so as to maintain the interface at a substantially constant pre-determined level.
The various froth streams are combined, deaerated, diluted with naphtha, and then centrifuged, to remove contained water and solids. Centrifuging involves passing the deaerated and diluted froth through two stages of centrifugation, using roll and disc centrifuges.
As indicated, the various froth products (PSV, TORV and secondary) contain water and solids as contaminants. It is the concentration of these contaminants that can vary widely, depending on the grade of the oil sand originally fed to the process.